Cust has two plinth heaters controlled by individual electronic timers.
The timers are sited right next to each other.
Timer A will start up fine when programmed.
When timer B clicks to start up.... BOTH controls (Timer A & Timer B) reset to blank screen.
The same occurs vice-versa.
Circuit does not trip and boost function is fine.

Now they probably aren't the most expensive timers, and must be the timers' electronics are "confusing" each other.. but that's all I could remark to cust.

If it is EMI then try fitting a piece of metal capping or trunking lid between the two timers with a fly lead and croc clips to an earthing point.....

Legh

I thought about suggesting that but due to the close proximity and mains voltage involved, it may not be a safe permanent solution?

Absolutely, I can imagine the disastrous aesthetic effect of a sheet of metal sticking out between two plastic timer controls with a croc clip attached.
If as OldSparky has stated that the programmers are rubbish to begin with then it would probably be worth investing in something more professional.
However, it would be satisfying to know a solution.